Current methods to produce hydrogen by steam methane reforming (SMR) usually require energy intensive processes. In conventional operations where a chemical solvent is employed to remove carbon dioxide from the reformed gases, the steam to carbon ratio (S/C) for the SMR reaction is set at a high ratio, in the order of 6/1 or somewhat higher, thus requiring a relatively high energy to generate the required steam. Also, the known chemical solvent processes typically require a comparatively high energy input for regeneration of the CO.sub.2 -laden solvent. On the other hand, if a conventional pressure swing adsorption (PSA) system is employed to remove CO.sub.2 from the reformed gas, hydrogen recovery is low, and the CO.sub.2 recovery and purity are also low thus requiring a larger quantity of natural gas feed to the SMR operation and employing larger equipment in the reforming areas for a given hydrogen production capacity.
A typical prior art system employing chemical solvent for removal of CO.sub.2 from the hydrogen-rich gas produced by steam-methane reforming (SMR) generally comprises a primary reformer, with provision for waste heat recovery, followed by reactors for high and low temperature water gas shift reaction to convert contained CO to CO.sub.2, according to the equation: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 (I)
Usually a methanation reactor is also employed for removal of residual carbon oxides by hydrogenation, according to the equations: EQU CO+3H.sub.2 .fwdarw.CH.sub.4 +H.sub.2 O (II) EQU CO.sub.2 +4H.sub.2 .fwdarw.CH.sub.4 +2H.sub.2 O (III)
In such conventional SMR operations employing chemical solvent for H.sub.2 /CO.sub.2 separation the steam to carbon ratio (S/C) in the feed to the reformer is set high (typically about 6.5) to provide a low methane concentration in the reformer effluent and to provide sufficient heat to drive the reboiler operation employed. The concentration of methane in the reformer effluent is about 2.75 mol% on a dry basis. The reformer effluent is cooled (as to about 685.degree. F.) before introduction into the high temperature shift convertor and the product from the high temperature shift reaction further cooled (as to about 425.degree. F.) for introduction into the low temperature shift convertor. The effluent from the low temperature shift reaction is sent to a chemical solvent absorber system for removal of CO.sub.2.
Typical solvent systems for CO.sub.2 removal by absorption from a hydrogen stream employ potassium carbonate solution or monoethanolamine (MEA) or mixtures of these; see, for example, U.S. Pat. No. 3,563,695. The solvent is regenerated by heating the drive off absorbed CO.sub.2, which is vented. The effluent hydrogen stream leaving the absorbent system may contain up to about 0.1% CO.sub.2 which is converted to methane by methanation. The methanation effluent will typically comprise on a dry basis about 96% hydrogen containing small amounts of methane and nitrogen as impurities.
A typical PSA system for removal of CO.sub.2 from a hydrogen-rich gas stream produced by SMR generally comprises primary reformer (SMR) and shift convertors as in the system previously described. Since a higher methane leakage can be tolerated in systems employing PSA for CO.sub.2 /H.sub.2 separation, the methane concentration in the stream leaving the primary reformer may be in the order of 8.25 mole% (dry basis). Following high and low temperature shift conversion the stream is cooled (to about 100.degree. F.) before introduction into the PSA unit. In the conventional PSA unit for CO.sub.2 removal from a hydrogen-rich stream a typical reject stream may comprise 19.8% methane, 47.8% CO.sub.2, 0.1% N.sub.2, 1.8% CO and 29.4% H.sub.2, and is generally employed as fuel in the steam reformer furnace. Hydrogen of 99.99+% purity is obtained as primary effluent from the PSA unit constituting about 85% of the hydrogen content of the stream entering the PSA adsorber.
Prior art PSA systems for bulk separation of hydrogen by selective adsorption of contained oxides of carbon and/or hydrocarbon gases from mixtures containing these are described in U.S. Pat. No. 4,077,779 and other patents therein cited. The '779 patent describes adsorptive separation of CO.sub.2 from a gas mixture also containing hydrogen or methane in a unit comprising five or more columns containing selective adsorbent, operated in parallel in a repeated cycle, employing in sequence in each column the steps of (1) adsorption, (2) rinsing with secondary component, (3) depressurization of the rinsed column, (4) purging the column with air or inert gas followed by (5) evacuation to desorb the column and finally (6) repressuring the column for repetition of the cycle. The described operation utilizes two rinse or purge steps, one at high pressure using secondary product gas recovered from the gas mixture being separated, and another at low pressure using air or inert gas from an outside source.
Other patents relevant to recovery of purified hydrogen from a steam methane reformate by PSA are listed below.
U.S. Pat. No. 3,150,942 describes purification of a crude hydrogen gas stream produced by methane steam reforming, wherein the impure reformer effluent is passed through two adsorbent beds in series. In the first bed type 13X molecular sieve is employed to remove all the water vapor and most of the CO.sub.2. In the second bed 4A or 5A type molecular sieve is employed to adsorb remaining impurities (chiefly CO) so as to yield a hydrogen product having a maximum of 0.2% impurities and being free of water vapor and oxides of carbon. Regeneration of both adsorbents is carried out by passing previously purified hydrogen as purge gas through both adsorbent beds in reverse flow and further desorption of the purged beds by heating. In certain preferred operations described in the patent, the initial steam reformate is cooled and treated with water vapor in a catalytic converter for oxidation of the CO content, yielding additional hydrogen and CO.sub.2.
Among various other gas purification processes disclosed in U.S. Pat. No. 3,176,444 using PSA is an example (FIG. 6 of the patent, and column 28) for removal of CO.sub.2 from a crude hydrogen stream using activated carbon adsorbent. The pressurized crude feed is passed through a dehydrator and then through a bed of activated carbon wherein carbon oxides are adsorbed. Part of the effluent is employed in partial repressuring of a companion adsorbent bed. On termination of the adsorption stroke the adsorbent bed is subjected to co-current depressuring to remove voids gas, until a preset intermediate pressure level. Further desorption is then had in countercurrent direction until the bed is just above atmospheric pressure level. Following the desorption steps the bed is repressured with collected hydrogen product.
In U.S. Pat. No. 3,430,418 the removal of impurities from a hydrogen-rich gas stream is described, wherein each of the adsorbent columns contains a first layer of activated carbon adsorbent and a second layer of zeolitic molecular sieve (calcium zeolite A). The activated carbon section selectively removes water and CO.sub.2 from the feed gas and the zeolite selectively removes CO and CH.sub.4. In a six step cycle including co-current and countercurrent depressuring followed by purging of the bed at atmospheric pressure, 76.5% of the feed hydrogen was recovered in the product.
U.S. Pat. No. 3,788,037 discloses a PSA operation applicable to recovery of hydrogen from a mixture such as that of a steam methane reforming operation, employing a low pressure purge step in the designed PSA cycle. In the specific example disclosed in the patent, hydrogen recovery was approximately 70% by volume of the hydrogen content of the feed.
While the adsorption system described in U.S. Pat. No. 4,000,990 is particularly directed to recovery of methane from landfill gas, the patent indicates that the process therein described can be utilized for upgrading a hydrogen-rich stream containing oxides of carbon and a small amount of methane. In the described process of the patent the feed gas is subjected to pretreatment in a thermally regenerated adsorbent bed followed by a pressure swing adsorbent unit. The PSA unit is operated in a designed sequence consisting of (a) adsorption at superatmospheric pressure while collecting unadsorbed effluent, followed by regeneration of the impurity-laden bed by (b) countercurrent venting to about atmospheric pressure level, (c) evacuation of the vented bed to subatmospheric level to effect desorption, and repressuring the bed to superatmospheric level with part of the primary effluent being charged countercurrently into the bed.
In certain known prior art processes, such as U.S. Pat. No. 3,479,298, methane-containing gas is subjected to a two-stage reforming operation, wherein following a primary reforming by reaction with steam, the obtained hydrogen-rich reaction product is subjected to secondary reforming by reacting the previously unreacted methane therein with oxygen. Thus, the methane (and other possibly contained hydrocarbons) undergo principal reactions as indicated by the equations below: EQU Primary: CH.sub.4 +2H.sub.2 O.fwdarw.CO.sub.2 +4H.sub.2 (IV) EQU Secondary: CH.sub.4 +O.sub.2 .fwdarw.CO.sub.2 +2H.sub.2 (V)
Among the objects of the present invention are to provide a process for efficient generation and recovery of hydrogen by reforming of a methane-rich gas stream under conditions of low capital and operating costs, and whereby the capacity of an existing SMR system can be beneficially extended.